1. Technical Field
The present invention relates to a light emitting diode (LED) driving circuit, and more particularly to an LED driving circuit capable of reducing a capacitance value requirement for a capacitor in the LED driving circuit and a cost of the LED driving circuit.
2. Related Art
FIG. 1 is a block diagram of an LED driving circuit in the prior art. The LED driving circuit includes an AC power source 1, a rectifying device 2, a driving chip 3, a switch 4, and a load circuit 5. An implement of the rectifying device 2 is a bridge rectifier for receiving external AC power from the AC power source 1, converting the external AC power into a half-wave or full-wave driving AC power, and outputting through a rectifying output terminal 2a. 
The driving chip 3 at least includes a receive pin 3a, a transmit pin 3b, a ground pin 3c, and a feedback pin 3d. The receive pin 3a is electrically coupled to the rectifying output terminal 2a and electrically coupled to one terminal of the load circuit 5. The transmit pin 3b is electrically coupled to the switch 4 and outputs a driving signal to set the switch 4 to on or to off. The ground pin 3c is indirectly or directly electrically grounded to obtain a zero potential reference. The feedback pin 3d is used for obtaining a potential to monitor the operation of the load circuit 5. The switch 4 has a first terminal point 4a and a second terminal point 4b. The first terminal point 4a is electrically grounded through a voltage dropping resistor 6. The feedback pin 3d of the driving chip 3 is electrically coupled to the first terminal point 4a to detect a potential of the first terminal point 4a, so as to determine whether the switch 4 is set to on or set to off.
The driving signal provided by the driving chip 3 is a pulse width modulation (PWM) signal for setting the switch 4 to on, so that the load circuit 5 obtains electrical power from the rectifying output terminal 2a to drive an LED 5a. Meanwhile, a magnitude of an average current received by the LED 5a may be changed by adjusting a pulse width and a pulse frequency, so as to change a brightness of the LED 5a. 
The potential of the transmit pin 3b of the driving chip 3 is required to be maintained above a threshold potential, so as to keep the driving chip 3 to work normally. When the rectifying device 2 is a bridge rectifier outputting a half-wave or full-wave driving AC power, a rectified waveform period thereof is a half-wave period or full-wave period. In the rectified waveform period, the potential rises from zero to a peak value and then drops from the peak value to zero. Since the rectified waveform period includes a time period in which the potential of the output of the bridge rectifier is higher than the threshold potential and a time period in which the potential the output of the bridge rectifier is lower than the threshold potential, the potential of the transmit pin 3b cannot be continuously maintained above the threshold potential. Therefore, for the rectifying device 2 such as a bridge rectifier outputting a full-wave or half-wave driving AC power, the LED driving circuit has to be modified.
FIG. 2 is a block diagram of a modified LED driving circuit.
The modified LED driving circuit further includes at least one balancing capacitor 7′ connected a rectifying output terminal 2a′ of a rectifying device 2′ to a ground wire. A balancing capacitor 6′ is a high-pressured electric capacity having high capacitance value.
When an output potential of the rectifying device 2′ is higher than a threshold potential, a current output from the rectifying output terminal 2a′ drives the driving chip 3′, charges the balancing capacitor 6′, and drives an LED 5a′ through a load circuit 5′ to emit light at the same time.
When the output potential of the rectifying device 2′ is lower than the threshold potential or the output potential is zero, the balancing capacitor 6′ is discharged to maintain a potential of a transmit pin 3b′ of the driving chip 3′ above the threshold potential so as to maintain the operation of the driving chip 3′.
When the balancing capacitor 6′ is discharged, a current output by the balancing capacitor 6′ is transmitted to the load circuit and consumed thereby. In order to avoid rapid drop of a voltage difference between two terminals of the balancing capacitor 6′ and prevent the potential of the transmit pin 3b′ of the driving chip 3′ from being lower than the threshold potential, the balancing capacitor 6′ must have an extremely high capacitance value, or even multiple balancing capacitors 6′ are required to be connected in parallel at the same time.
The balancing capacitor 6′ having a high capacitance value incurs an extra manufacturing cost. Meanwhile, a capacitor having a high capacitance value also has a large volume and occupies extra space, so that a volume of an LED illumination device cannot be effectively reduced.